The present disclosure relates generally to compositions and methods for screening for diseases or disorders associated with a deficiency in frataxin. More particularly, the present disclosure relates to methods of screening for a disease or disorder associated with a deficiency in frataxin, as well as methods of screening for therapeutic agents for use in treating a disease or disorder associated with a deficiency in frataxin, or damage from reactive oxygen species to mitochondrial proteins affecting the respiratory chain, or increased mitochondrial protein acetylation affecting mitochondrial function, as well as method of screening for therapeutic agents for use in treating a disease or disorder associated with a deficiency in frataxin.
Friedreich's Ataxia (FRDA) is an autosomal recessive mitochondrial disorder caused by a homozygous triplet nucleotide repeat (GAA·TTC) expansion in intron 1 of the FXN gene located on chromosome 9q21.11. This intronic expansion causes impaired transcription of the FXN gene and, consequently, a pathological deficiency of the FXN gene product, frataxin. Frataxin is targeted to the mitochondrial matrix, where it is known to act as an iron-binding protein and participates in the proper assembly and function of iron-sulfur cluster (ISC) dependent proteins including complexes I, II, and III of the respiratory chain and aconitase of the tricarboxylic acid (TCA) cycle. Thus, frataxin deficiency severely compromises both cellular respiration and overall mitochondrial function leading to energetic stress and ATP deficiency. Although patients develop multisystem disease including early spinocerebellar degeneration, ataxia, and diabetes, the primary cause of death is heart failure for nearly 85% of those afflicted. Similarly, although the phenotypes of the neuron-specific enolase (NSE) and muscle creatine kinase (MCK) Cre conditional mouse models of FRDA differ, both models develop a fatal cardiomyopathy and impaired activity of iron-sulfur cluster-dependent respiratory complexes consistent with the human disease.
Recent work has demonstrated that lysine acetylation is a highly conserved and abundant post-translational modification within mitochondria that is responsive to nutrient availability and may contribute to the physiological adaptations of reduced caloric intake. Multiple investigations have demonstrated a role for reversible mitochondrial enzyme deacetylation and, specifically, the NAD+-dependent deacetylase SIRT3, in the regulation of fatty acid oxidation, the TCA cycle, electron transport via respiratory complexes I and II, and overall oxidative metabolism. SIRT3-mediated deacetylation has recently emerged as a major mechanism regulating the activity of mitochondrial oxidative and intermediary metabolism. SIRT3 is also uniquely poised to respond to the flux of mitochondrial NAD+ and NADH, which is determined, in large part, by the capacity of the respiratory chain to oxidize NADH. This capacity is severely decreased in FRDA, as well as in other mitochondrial defects such as cytochrome c oxidase (complex IV) deficiency, causing an accumulation of NADH and, consequently, a redox state of perceived nutrient excess.
Defects in cellular respiration may be inherited as mitochondrial disease, or acquired over a lifetime via somatic mutations, and are linked to many conditions including neurodegenerative disease, diabetes, heart failure, cancer and in the aging process in general. The involvement of cellular respiration in numerous common human pathologies emphasizes the need for greater understanding of the pathophysiological processes that occur in response to respiratory chain compromise. Accordingly, there exists a need to develop biomarkers of diseases or disorders associated with a deficiency in frataxin.